This invention relates to the pyrolytic deposition of a uniform film of silicon dioxide onto a heated substrate. More particularly, this invention relates to the deposition of silicon dioxide on a vertically positioned substrate by reacting silane with oxygen in an evacuated system.
Silicon dioxide (SiO.sub.2) is a dense, chemically inert, dielectric material of extreme hardness, low thermal conductivity and high resistance to molecular diffusion. These properties have made silicon dioxide an attractive and valuable material for a wide range of applications. For example, in the fabrication of semiconductor devices, silicon dioxide is useful in forming the intermediate dielectric layers of multi-layer metal devices and in forming the final passivation coating for a completed device. Further applications for deposited layers of doped silicon dioxide are as diffusion sources for doping silicon and for reflow glass processes. Relatively thick layers of silicon dioxide are also used to form the field oxide layer of MOSFET semiconductor devices.
Although prior art processes for the deposition of silicon dioxide are functional in certain applications, these processes present drawbacks in other areas. In general, it has been found to be extremely difficult to deposit silicon dioxide onto semiconductor substrates in a manner that will allow good growth rates, uniform deposition and a high quality coating in an economical process in which a large quantity of semiconductor substrates can be coated at the same time.
One class of prior art systems can be categorized as "one atmosphere" systems in which silicon dioxide is deposited by oxidizing silane on the surfaces of semiconductor substrates which are heated to 350.degree. to 450.degree. C. The substrates are horizontally mounted inside an unpressurized chamber (commonly called a reactor) whose walls are maintained at approximately room temperature. Although the formation of silicon dioxide tends to be localized at the heated surface of the substrate, a gas phase reaction is spontaneous at room temperature and must be moderated by diluting the reactance with nitrogen. Some of the problems associated with this type of deposition are: (1) The gas phase reaction cannot be totally eliminated so particles of silicon dioxide thus formed can be carried to the surface of the substrate causing pinholes in the silicon dioxide coating. (2) The substrates are mounted horizontally so that the particles that fall off the reactor walls land on the substrate surface, again causing pinholes. The reactor walls must be cleaned frequently to minimize this problem. (3) The method used to heat the substrates, which typically involves placing the substrates on a heated surface inside the reactor, makes it difficult to obtain the accurate control of substrate temperatures required to maintain the desired coating composition and deposition rate. (4) The flow rates of nitrogen, oxygen and silane must be controlled throughout the entire deposition to maintain a uniform coating necessitating expensive and complicated gas flow panels.
Another class of prior art systems is partial vacuum deposition systems. Silicon dioxide is deposited by oxidizing silane on the surfaces of semiconductor substrates which are heated inside an evacuated chamber. For certain applications, other gases such as phosphine are included with the silane to deposit "doped" silicon dioxide. Typically, heating is accomplished by adapting a diffusion tube to allow evacuation and then using a standard diffusion furnace to provide well controlled inductive heating. The basic principle or concept involved in this method is to use reduced pressure rather than a nitrogen diluent to moderate and control the gas phase reaction. Because no nitrogen diluent is required, the total gas flow of the system is much lower and any gas phase reaction particles which do form in the vacuum system are less likely to be swept through the system. In such a system, wafers can be mounted vertically and thus do not catch particles falling from the walls of the tube. Further advantages of this method derive from the fact that the art of temperature control has been highly developed in tube type diffusion furnaces where temperature zones of more than 20 inches can be controlled within .+-.1.degree. C.
In spite of the above advantages obtained with partial vacuum deposition systems, the implementation of these systems to provide high volume deposition of silicon dioxide has proven difficult. Using conventional methods of delivery of silane and oxygen gas to the evacuated chamber and conventional methods for evacuating the chamber, it has been found that silicon dioxide tends to deposit nonuniformly on the silicon substrates within the chamber. This nonuniform deposition takes the form of a thickened ridge or ring around the periphery of the substrate which makes the substrates unsatisfactory for further process steps. In prior art systems, the only method for avoiding this nonuniform deposition has been to severely restrict the number of substrates deposited at one time so that they can be widely spaced with respect to each other inside the evacuated chamber and further to greatly reduce the flow rate of the reactant gases with the effect that the resultant deposition rate is very low. These limitations severely restrict the process throughput and accordingly increase the cost, thereby severely limiting the applicability of this method of silicon dioxide deposition for high volume production operations.